Who's Shooting at Whom?

The first thing that should be pointed out is that canons don’t shoot things, and cannons have no books in them.  The two words do (probably) descend from the same word, the Greek word kanna, which means a reed.  Reeds are straight, which suggests an authoritative standard, while they are also hollow tubes, which suggests tubes that shoot things.  But we’re talking about canons.

And canons do involve books (usually, anyway, although it can also be used for other collections like the Star Wars movies).  It’s a body of work that everyone accepts as authoritative.  For example, J.K. Rowling’s Harry Potter series has seven books.  They are all written by one author, about the same topic, and were intended from the beginning to form one whole.  No one asserts that one of those seven does not belong in the Harry Potter canon.  On the other hand, there are other books, written by the same author, about topics related to the original seven, and opinion among the fans is more divided about them.  Some would accept Fantastic Beasts and Where to Find Them as canonical and others do not, or give them lesser authority.  And hardly anyone thinks that Harry Potter & the Cursed Child should be canon, even though it too was written by Rowling and she has said it should be considered part of the canon. 

So a canon is a collection that is widely regarded and accepted as authoritative, but rarely declared so by any specific person or group with authority to decide what is or is not canon.  Its makeup does not necessarily even depend on the author’s intentions or opinions: rather, a canon reflects the opinions of many people, often over many years.  It's not really a popularity contest, however: it’s more accurate to say that a canon reflects the opinion of everyone, reached more or less independently, about which books should be part of the whole.  Doubtful books tend to fall out, merely because many people do not accept them and the rest find that they must use the other universally-accepted books if their arguments are to reach the widest audience.  Thus when a canon finally becomes settled it is not accepted by just 51% of people, but by something much closer to 99%.  In short, a canon becomes settled over time, not declared by an authority.